Controlling kerr-cells



May 18, 1943 c, BECKER 2,319,289

CONTROLLING KERR-CELLS Filed Feb. 19, 1941 2 Shets-Sheet l May 18, 1943 c, BECKER 2,319,289

CONTROLLING KERR-CELLS Filed Feb. 19, 1941 2 Sheets-Sheet 2 [n renfor': CZer/Je m2 Fee/("e r".

Patented May 18, 1 943 comraoumo nan-ems Cariheins Becker, Beriin-WiimetsdrL Germany} vested in the Alien Property Custodian Application February 19, 1941, Serial In Germany February 14 Claims. (01. 119-10013) This invention relates generally to methods of photographically recording sound and more particularly to a method of controlling light-beam modulators such as Kerr-cells when making halfwave intensity sound records and to a novel type of Kerr cell useful in this and in other methods of sound recording.

In the making of old style fullwave intensity sound records using Kerr-cells as modulators of the light beam, sound traces were obtained which were characterized by isolated sound strips spaced in the direction of the sound trace. In these old style intensity records these strips of sound trace caused no trouble, hence the cause of the production of the strips or methods of preventing" their formation have not been investigated.

When making half-wave intensity recordshowever, it was found that absolute uniformity and continuity of the sound traces was essential. The presence of these isolated strips resulted in an unsymmetrical intensity ratio of thetwo halfwave traces during scanning and, at each point of cut of the negative, in a lack of symmetrywhich as a practical matter could not be satisfactorily compensated. Moreover, the presence of the strips prevented the quantitative adjustment of the density of the sound trace as a function trobenzene, together with impurities or by-products contained therein, is decomposed and in this instance too the electrodes become coated by the decomposition products deposited thereon.

To suppress direct current components of the Kerr-cell operating voltage as is accomplished in accordance with the present invention, at least one of the cell electrodes, it has been suggested,

. should be coated with a thin homogeneous insulating blocking layer. This method is disadvanageous, however, due to the difficulty presented by the production of a blocking layer of sufficient uniformity. A further disadvantage is the loss of light 'caused bythe blocking layer. which is due to the finite conductance of the nitrobenzene and the voltage drop occurring thereby in the blocking layer.

Another method of. minimizing effects due to direct current components of the Kerr-cell operating voltages consistsun supplying the electrodes of the Kerr-cell with biasing or half-wave voltof the modulation which is absolutely necessary for undistorted half-wave intensity recording. This quantitative density adjustment is necessary for obtaining the desired density range necessary for proper half-wave intensity recording.

In accordance with the present invention, a method is provided for suppressing thesev strips in the sound trace in the making of half-wave intensity records by suppressing the direct currents in the Kerr-cell operating voltage. The invention is based upon a fundamental discovery made during exhaustive physical researches in this field using Kerr-cells provided with electrodes polished free of surface irregularities and optically homogeneous. It was found that direct currents in the cell operating voltages caused the formation of these strips. In the same manner that an electrolyte is decomposed if a direct current flows through the same, so is a Kerr-cell medium electrolyzed by direct current components of the Kerr-cell operating voltage flowing therethrough. If the electrodes consist of a metal that can react with the electrolyte under these conditions, they become coatedwith decomposition products which substantially alter the course of the electric field between the electrodes. In the case of inert or non-reactive electrodes, the

ages in form of symmetrically modulated high frequency alternating currents. In this manner electrolytical decomposition of the cell medium is obviated.

For special purposes it has been proposed (German Patent 471,720) to control the Kerr-cell with low frequency modulated high frequency currents, for instance for making records in luminous or light telephony, picture telegraphy. etc,

a This method is said to possess the advantage that it allows the use of a sensitive high frequency amplifier in the receiver. However, this factor I is of no importance with relation to the subject matter of the present invention, because high frequency voltages GIQ not recorded in the sound 7 trace. I

The control of a Kerr-cell with high frequency currents for the purpose of suppressing direct current components has not yetbeen proposed.

The present invention is based in part upon this novel principle.

To obtain as wide a light range of modulation as possible. it is preferable that the modulated high frequency alternating potential usedv in operating the'cells havethe form of a symmetrical rectangle so that the eil'ective value of the modulated voltage is equal to its amplitude value. In practice, however, it has been found than it is not absolutely necessary to supply all the voltages across the.Kerr-cell (half-wave voltage, biasing voltage, and compensating voltage) in the form of modulated high frequency voltages. It is'customary, therefore, for reasons of Kerr-cell medium, which may be for instance nicircuit simplification, to supply to the electrodes of the cell only the biasing voltage as a high frequency voltage, the half-wave voltages and the threshold voltages being supplied in the ordinary manner. In this manner oxidation or hydration of the electrode surfaces is largely suppressed and electrolytical decomposition, which would normally be caused by the half-wave voltages and the threshold voltages, is prevented by the high frequency biasing voltage.

A disadvantage of using modulated high frerecording sound voltages are applied across the primary winding of the transformer 8 and, at the quency voltages with Kerr cells is the high heating which results, causing considerable loss of the nitrobenzene or other cell medium which in turn substantially alters the light ratios of the cell.

To avoid these difiiculties and disadvantages inherent to the last described method of operating a Kerr cell, it is proposed in accordance with this invention-to use a suitable commutator for continuously reversing the polarity of the electrodes of the cell. The frequency of the polarity change preferably is chosen sufflciently low to be below the frequency range to be recorded and the period of interruption during the reversal should be so brief that records upon the sound trace are not produced. For example, the polarity reversal may occur at the rate of'about to 50 seconds, whereas'the period of interruption during reversal of polarity should not be of longer duration than one twenty thousandth (.00005) of asecond. If all these requirements cannot conveniently be satisfied, the reversal of polarity of both half-waves is efiected exactly simultaneously and symmetrically whereby, in reproducing the sound trace recording, interruptions effected upon the two half-wave traces are dynamically compensated by'the push-pull circuit or are masked by the sound record.

For the purp se of avoiding interruptions in the sound traces, it is also desirable to include in in each of the two Kerr-cell circuits a high resistance (for instance a resistance of 100,000

secondary side of the transformer, these alternating voltages are separated into half-wave components by rectifiers 4 and 5 which derive their operating currents from tapped resistances l and 2. For recording the half-wave intensity records, a Kerr-cell Ii is used which comprises two pairs of electrically independent plates a, b and c, :1 respectively. The voltage across the pair of plates a, b is composed of the half-wave voltages occurring at the resistance 2, the Kerrcell biasing voltage 0 and the compensating voltage supplied by the amplifier tube i to the resistance l0 which compensates for non-linearity of the photographic emulsion characteristic curve. These three voltages are connected in series in such a manner that the Kerr-cell biasing voltage, 8 also serves as anode voltage for the amplifier tubes 1 and 6. In like manner, the voltage across the pair of plates c, d is composed of the half-wave voltages occurring at the resistance i, th Kerr-cell biasing voltage 8 and the compensating voltage supplied by the amplifier tube 6 to the resistance 8. The two amplifier tubes 6 and I are arranged in push-pull connection and are supplied with their control voltages by means of tappings on the resistances l and 2 respectively. The connection and the point of operation of the amplifier tubes 6 and I are so chosen that the half-wave voltages occurring at the resistances 9 and 80 respectively are added.

- With increasing half-wave voltages at the resistohms) which is shunted after reversal of the voltages, thus protecting the cell against voltage surges.

A substantial advantage of this novel method of Kerr-cell control is that it may be used in connection with existing types of recording apparatus without material modification as it is merely necessary to connect the Kerr-cell terminals across a commutator or equivalent device.

In practicing the half-wave sound recording method according to this invention, it has been found that the electrodes of the Kerr-cell must be ground optiqally plane and be of uniform rede etion capacity to provide uniform sound traces. For this purpose, it is preferable to use plane ggound porcelain or quartz electrodes provided with a conductive metal surfacing layer, preferably of an inert corrosion-resistant metal such as a metal of the platinum group. It is important that the metal selected have a minimum absorption capacity in the wave length range of light used with, the cell as otherwise the emciency of the cell is considerably reduced. Research also indicates that, to minimize electrolytic reactions occurring within the cell, it is necessary to form the casing of glass or quartz and to make the leads and compression springs of the electrodes of an inert corrosion-resistant metal.

In the accompanying drawings Figs. 1 to 4 inclusive are circuit diagrams illustrating applications of the principles of this invention.

In the circuit diagram shown in Fig. 1, the

ances l and 2 respectively the voltages occurring at the resistances 9 and i0 respectively are rendered smaller in proportion to the resistances l and 2 respectively and in this manner effect a compensating effect for non-linearity of the foot of the photographicemulsion characteristic curve of the material used for making the record.

In accordance with the present invention, the three serially connected voltages 2, 8, l0 and l, 8, 9 respectively above discussed are not directly supplied to the pairs of plates a, b and c, d respectively of the Kerr-cell H, but instead a commutator i9 consisting of the devices H, l2, I3, is and I5, l6, I1, I8 respectively is connected in the circuit to effect reversal of polarity. The devices ll to 18 by means of which polarity reversal is accomplished consist of small circular discs, one half of each of which is electrically conductive, the other half being electrically insulative, each wiped upon the circumference by a current conductor. These devices II to I! for the reversal of polarity are rigidly mounted upon a shaft in a manner such that the conducting and insulating halves are exactly staggered with regard to each other substantially as illustrated. By means of these polarity reversing devices each of the plates a, b of the Kerr cell it are alternately connected to the ends of'the resistances I and 8. The commutator is rotated at a speed preferably of the'order of 20 to 50 revolutions per 'second whereby the individual polarity reversals justment of the rate of polarity reversal is simplifled by the fact that the voltage for each pair of electrodesis derived from across two voltage dividers or potentiometers each formed by two resistances connected in series. Each of the voltage dividers is adjustable with regard to value, whereby one electrode of each pair of electrodes of the Kerr-cell is connected to the centre point tap of one of the pair of potentiometers, the resistances being so controlled that during one half the control period the resistance of the second potentiometer arranged nearest to one or the voltage poles has its lowest and the other two resistances have their highest values, whereas in the second half of the conrtrol period the relationship is reversed.

It is particularly preferred, in practicing this invention, to use as controllable resistances gridcontrolled electron tubes, the grids oi which are supplied with rectangular control voltages, whereby the control voltages for the two electron tubes of a potentiometer and for the electron tubes connected nearest to the same voltage poles of the Kerr-cell are displaced about 180 in phase with regard to each other. To prevent grid currents it is necessary that a protective resistance be arranged in the grid circuit of each of the electron tubes.

Instead of grid-controlled electrontubes the circuit according to this invention may be modifled by using as controllable resistances, magnetically controlled electron tubes (magnetrons) or grid-controlled gas-filled ionic-tubes (currentrons). In all cases care must be taken that the minimum resistances of the devices used are small with regard to the resistance of the Kerrcell-so that no distortions in the sound record are eilected by a voltage drop across these resistances.

In the circuit of Fig. 2, the Kerr-cell ll consists of the two pairs of plates a, b and c. d operated by twohali-wave voltages. For the sake of clearness the connections or one pair of plates c, d, i. e., for one hall-wave voltage only, are shown, as the connections for the second pair of plates a, b are exactly the same. The hall'- wave voltages are produced in exactly the same manner as indicated in Fig. l and, therefore, need not be described in detail here. The voltvoltage is continuously reversed in the manner dlsclosed'above.

Across the voltage at the points A, B two bridge isassumedthattheplus poleotthebiasingvoltagesourceisconnectedtothepointfi. Also connectedtothepointBaretheanodesofthe asiaaao age serving to control the plates c,'d is supplied tothepointsAandBandthepolarltyofthis' tubes ilandlfiandtothepointathecathodesofthetifiies i2 and i5. Thecathodeofthetube llisconnectedwiththeanodeofthetube iland tothisconnectiontheplatecottheKerr-cellis .camected. Inlikemannerthecaflmdeoithe tuhellisconnectedwiththeanodeotthetube 'llandtheplatedofthekerr-celliaconnected hthkconmctlm; Lvoltageissupvoltage, because the two rectangular voltages are displaced about 180 in phase with regard to each other. Thetubes I I and I5 therefore, now have a very low inner resistance, whereas the tubes l2 and II have a very high inner resistance. The plate c of the Kerr-cell, therefore, practically is connected to the point .A, the plate d of the Kerr-cell to the point B. After halt a period the signs 01' the rectangle voltages change and the plate 0 is connected to the point B and the plate d-to the point A.

The more the control voltages approach an ideal rectangular voltage, i. e., the larger is the slope of their sides, the shorter will be the change-over period, thus minimizing interference with the sound track. To prevent distortions. care is to be taken that'the resistance of the electron tubes is very small with regard to the resistance of the Kerr-cell as long as the positive portion of the rectangular voltage is effective at these tubes.

It has been found that by the use of usual types of electron tubes, large distortions ofthe halfwave voltages acting upon the Kerr-cell occur. The extraordinarily large resistance of the Kerrcell prevents rapid dissipation of the half-wave potentials, and dissipation of these voltages by way of electron tubes also is not possible, be-

cause these tubes pass current in one direction only. e

To obviate the distortions thus produced, grid controlled electron tubes, the anodes or which are electron emitting electrodes, are used as con other cathode with the grids connected together.-

Flg. 3 is a diagrammatical illustration of this type of circuit. The Kerr-cell I is provided with the two pairs of electrodes a, b and c, d. The circuit is shown for the pair of plates 0, d only as the connection for the other pair of plates a, b corresponds exactly thereto. The voltage serving to control the plates c, d is connected across the points A and B, and two potentiometers' connected in parallel are arranged. consisting of the electron tubes II. II and l2, l3. respectively, are

connected across the points A, B. Connected to thelinkage of the electron tubes i4 and I5 is the plates of the Kerr-cell Ii and to the linkage of the electron tubes [2 and ii the plate (1. The

, electron tubes l2, l3, I4 and Ii are provided with isinchidedsuehastheresistancesflJflfland ltwhlcharearrangediorgri rents whmeverpositivelrldvoltagesoccm'.

Thiacircuitoperatesastollowsnurlngapre- -cleterminedperiodoftimethetuhes Band llare miedwitharelativclyhlahpdflvegrid voltage,whereassimultaneomlythatubes llandll directly or indirectly heated electron-emitting electrodes of the same kind that serve as cathodes and anodes, whereby the tubes are current permeable in both directions and the half-wave potentials may be freely dissipated.

It has been found that the production of reetangular voltage" in phase about presents considerable dimculties because very high exactness is required it the voltages are not to be audible on the sound record. In particular it is necessary that the rectangular voltages displaced in phase about 180 must match each other with high exactness with neither over-lapping or form connectingsto In accordance with the present invention,rectangularvoltages displaced in phase about 180 maybe derived from the anode circuits of two of the amplifier tube It is connected to the nega-' amplifier tubes. the grid biasing voltages of which alternately are rendered so highly negative that no anode current flows in the amplifier tube connected therewith. The two alternately highly negative grid biasing voltages for the two amplifier tubes are derived from an intermittently interrupted circuit consisting of ohmic resistances and a voltage source. By this method the operation of the change-over switch occurs satisfactorily, supplying the rectangular voltages displaced in phase in exact successive order.

A simple connection for performing this switching operation consists in connecting the minus pole of the grid biasing voltage which determines the operative point of the amplifier tubes, through a periodically operating circuit interrupter to the centre of a potentiometer, consisting of, two resistances across which a source of voltage at least twice the negative grid biasing voltage required for blocking the anode current is applied. The grid of one of the amplifier tubes is connected to the negative end of the potentiometer and the grid of the other amplifier tube to the middle of the voltage source. This circuit is illustrated in Fig. 4.

- Referring to the circuit of Fig. 4, the normal Operative point of the amplifier tubes it and ii is given by the constant grid biasing voltage i3. The negative pole of the grid biasing voltage is connected across the middle of a potentiometer formed by the two resistances H and IE-across which the voltage source it is connected by a periodically functioning interrupter it. The grid tive end of the potentiometer resistances M, ii, and the grid of the amplifier tube ll is connected to the middle of the voltage source M. The operation of this circuit is a follows. When the switch .H5 is in open circuit position, no'drop of voltage occurs at the resistances ii and I2 and at the grid of the amplifier tube is the grid biasing voltage it only is effective. The amplifier tube i6, therefore, now operates in its normal operative point and anode current flows. While:

the switch It is opened, a negative voltage, composed of the grid biasing voltage i3 and half the voltage i8, is connected across the grid of the amplifier tube 'i'i. This half of the voltage it! must be chosen so large as to be sumcient for blocking the anode current in the amplifier-tube ii.

When the switch i5 is in closed circuit position, a drop of voltage occurs at the resistances ii and 52. As the grid of the amplifier tube i6 is connected to the negative end of the potentiometer a negative biasing voltage will be effective at the grid of this tube, which biasing volt age is composed of the voltage i3 and the voltage drop at the resistance ii. The anode current through the amplifier tube is is blocked.

At the same time three voltages connected in series, 1. e., the biasing voltage i8, the voltage drop at the resistance i2 and the one half of the voltage it, are applied to the grid of the ampli fier tube ii. If the negative pole of the biasing voltage i3 is connected to a point of the potentiometer having the same potential as the point of the voltage source it connected to the grid of the amplifier tube ill, the voltage drop at the resistance i2 and the portion of the voltage Id connected across the grid compensate each other, and the grid biasing voltage is given by the voltage l3. For this reason the circuit elements are so chosen that the resistances H and ii are equal and that the grid of the amplifier tube ii! is connected to the middle of the voltage source M. The amplifier tube ll now operates in its normal operative point and anode current flows.

The periodically operating circuit interrupter i5 is one of the known automatic electrical switches of this type chosen to effect opening and closing of the contacts several times in the second.

Any desired number of galvanically independent rectangular voltages may be derived from the anode circuits of the tubes it and H by a corresponding number of low frequency transformers. The rectangular voltages derived from the anode circuit of the amplifier tube it are displaced about 180" with regard to the rectangle voltages derived from'the anode circuit of the ampiifier tube ll.

What I claim'is:

ii. In a method of controlling Kerr-cells when making records in half-waves intensity records, the steps of suppressing direct current generated by the Kerr-cell voltage by continuously reversing the polarity with regard to the voltages across said electrodes; by means of a suitable commutator, the frequency of reversal of polarity being chosen so low that it is below the frequency range to be recorded and electrolytically effective direct current is not yet generated, while said reversal of polarity of the voltages is effected in so short periods of time that no records upon the sound trace are produced hereby.

2. In a method of controlling Kerr-cells when making records inhalf-wave intensity records, the steps of suppressing direct current in the Kerr-cell voltage by continuously reversing the polarity with regard to the voltages, connecting each voltage determined for a pair of electrodes each across two potentiometers connected in parallel and each consisting of two serially connected resistances controllable in its value, whereby an electrode each of the pair of electrodes of the Kerr-cell is connected each to the middle of a potentiometer and the resistances are so controlled that during one half of the control period simultaneously the resistance of the first potentiometer arranged nearest the voltage pole and the resistance arranged nearest the other voltage pole have their lowest and the two other resistances have their highest value, whereas in the second half of the control period the proportions are reversed. k

3. A method as set forth in claim 2 in which, as controllable resistances, grid-controlled electron tubes are used the grids of which are supplied with rectangle control voltages, the control voltages for both the electron tubes of a potentiometer and for the electron tubes arranged nearest the same Kerr-cell voltag'e poles are displaced about 180 with regard to each other.

4. A method a; set forth in claim 2 in which, as controllable resistances, grid-controlled electron tubes are used the grids of which are supplied with rcctangle control voltages, the control voltagesfor both the electron tubes of a potentiometer and for the electron tubes arranged nearest the same Kerr-cell voltage poles are displaced about 180 with regard to each other and in which a protective resistance is arranged in the grid circuit of each electron tube.

5. A method as set forth in claim 2 in which magnetically controlled electron tubes (magnetrons) are used as controllable resistances.

6. A method as set forth in claim 2 in which trons) are used as controllable resistances.

'7. A method as set forth in claim 2 in which the lowest values of the controllable resistances are small with regard to the Kerr-cell resistance.

8. A method as set forth in claim 1 in which, as controllable resistances, grid-controlled electron tubes are used the anodes of which are electron-emitting electrodes.

9. A method as set forth in claim 1 in which as controllable resistances, grid-controlled electron tubes are used the cathode and anode of which are directly or indirectly heated electron-emitting electrodes of the same kind.

10. An electron tube for carrying out the method set forth in claim 8 in which the anode and cathode are directly or indirectly heated electron-emitting electrodes.

11. A modification of the method set forth in claim 8 in which, as controllable resistances, two electron tubes are used which are normally connected in parallel opposite to each other.

12. In a method of controlling Kerr-cells when making records in half-waves intensity records, the steps of using, as controllable resistances, grid-controlled electron tubes the'grids of which are supplied with rectangle control voltages and deriving said rectangle voltages from the anode circuits of two amplifier tubes the grid biasing voltages of which alternately are rendered so strongly negative that no anode current flows in the appertaining amplifier tube.

13. In a method as set forth in claim 12, the

steps of deriving the two alternately strong neg-- ative grid biasing voltages for the two amplifier tubes from an alternately opened and closed circuit consisting of ohmic resistances and a potentiometer.

14. A connection for carrying out the method set forth in claim 12 in which the minus pole of the grid biasing voltage determining the opera-- tive point of the amplifier tubes is connected to the middle of a potentiometer across which, by way of a periodically operating connecting and disconnecting switch, a source of voltage is arranged the voltage of which is at least double as large as the negative grid voltage necessary for blocking the anode current, and in which the grid of the one amplifier tube is connected to the negative end of the potentiometer, the grid of the other amplifier tube to the middle of the source of voltage.

CARLHEINZ BECKER. 

